Airbags for the driver's seat and the passenger seat for increasing the safety of automobiles are already becoming standard equipment. In recent years, the types and number of airbags mounted on automobiles have been increasing more and more, and side airbags and curtain airbags for side impacts, knee bags for protecting the lower extremities, and the like are being developed.
Methods of operating an airbag include chemical types in which an explosive chemical is used to generate a gas for expanding an airbag and hybrid types (including pressure storage types) in which a member referred to as an inflator (or an accumulator) is filled with a high-pressure gas for expanding an airbag. Chemical types were the first to be developed, and hybrid types were later developed due to a demand for an increase in the responsiveness of the expanding speed of airbags and adjustment of the gas pressure. In addition, a pressure storage type has been developed for curtain airbags which require a relatively long holding time. At present, the type which is used depends upon the performance demanded of the airbag, the location of installation, and the like.
An inflator for use in a typical hybrid type airbag (including a pressure storage type airbag) has a structure comprising a bottle member which is a short steel tube having both ends reduced in diameter by drawing, one end (the end to be connected to an airbag) of the bottle member being closed with a lid plate welded thereto, and an initiator (an airbag triggering unit) being secured on the other end of the bottle member. The interior of the bottle member is filled with a high-pressure inert gas. When a collision is sensed, the lid plate is opened either mechanically or by a rise in internal pressure, and the high-pressure gas inside the inflator is discharged all at once into the airbag so that the airbag is inflated.
Accordingly, a bottle member used for manufacturing a inflator for a hybrid type airbag (including a pressure storage type) is subjected to stress at a high strain rate in an extremely short period of time. Therefore, in contrast to a simple structural member such as a conventional pressure cylinder or line tube, this bottle member is required to have not only a high dimensional accuracy, workability, and weldability, but it is also required to have high strength and excellent burst resistance as well as high toughness. Automobiles may be used in cold regions, so burst resistance must be maintained at a low temperature of −40° C. or below.
Even with an inflator for a chemical type airbag having its interior filled with an explosive chemical, when gas is generated at the time of an impact, its interior reaches a high pressure. Recently, the generated pressure has been increasing with an improvement in the performance of chemicals, and burst resistance comparable to that for hybrid types has come to be demanded of bottle members for chemical type airbag inflators.
In the present invention, an airbag inflator bottle member means a steel member comprising a tubular body which has at least one end portion reduced in diameter and which is used for manufacturing a steel storage portion (namely, an inflator) for housing high-pressure gas and/or a propellant (an explosive chemical) for use in expanding an airbag or other passenger restraint system.
This bottle member has its interior formed into a sealed space by securing a lid plate and an initiator, or the like by welding or a similar method, and it is used as an inflator. The type of airbag can be a hybrid type (including a pressure storage type) or a chemical type. In the case of a hybrid type (including a pressure storage type), the interior of the inflator is filled with a high-pressure gas, and in the case of a chemical type, the interior of the inflator is filled with a propellant prior to sealing.
As stated above, increased strength, a reduced wall thickness, a decrease in size, and higher toughness are demanded of an airbag inflator bottle member.
A typical conventional process for manufacturing an airbag inflator bottle member comprises either (a) subjecting a steel tube (steel tube) as a material to be worked to quenching and tempering and then performing thereon drawing and stress relief annealing, or (b) subjecting a steel tube as a material to be worked to drawing followed by quenching and tempering of the tube. Subsequently, the steel tube is cut to a predetermined length, and one or both ends of the tube undergoes a reduction in diameter in order to allow parts such as an initiator to be fitted to the ends. Furthermore, the tube undergoes various types of working including piercing, local working such as spherical indentations, for example, fitting with a lid plate and an initiator by welding. See, for example, the following Japanese patent publications JP H08-325641 A1; JP H10-140250 A1; JP H10-140283 A1; JP 2002-294339 A1; JP 2003-201541 A1; and JP 2005-60796 A1.